Use of acetal polymers containing carboxyl groups in photosensitive compositions and lithographic printing plates
The present invention relates to binders and photosensitive compositions which are suitable for the production of printing plates.
Photosensitive compositions which are used in particular for high-performance printing plates now have to meet high requirements.
Essentially two approaches are adopted for improving the properties of photosensitive compositions and hence also of the corresponding printing plates. One is concerned with improving the properties of the photosensitive components in the mixtures (negative diazo resins, photoinitiators, photopolymers etc.), while the other is concerned with finding novel polymeric compounds (xe2x80x9cbindersxe2x80x9d) which are intended to control the physical and the mechanical properties of the photosensitive layers. Particularly the latter approach is of considerable importance for printing plates since the behavior in the development and printing process, such as, for example, developability, ink acceptance, scratch resistance and press life, is decisively influenced by the polymeric binders. The shelf-life and the photosensitivity of the materials are also influenced by such polymeric compounds.
In order to meet the extensive requirements, the polymeric binders have various structural elements which can affect individual properties differently. Thus, hydrophilic structural elements, such as for example carboxyl groups and hydroxyl groups, generally promote good developability of the photosensitive compositions in aqueous alkaline developers and in some cases ensure sufficient adhesion to polar substrates. Hydrophobic structural elements, on the other hand, complicate developability in said developers but ensure the good acceptance of the inks in the printing process, which is absolutely essential for printing plates.
Because of the wide range of requirements which the polymeric binders have to meet, there has been extensive work for many years on synthesizing, and optimizing the use of, these substances for photosensitive compositions (cf. for example H. Baumann and H.-J. Timpe: xe2x80x9cChemical Aspects of Offset Printingxe2x80x9d in J. prakt. Chem. Chemiker-Zeitung 336 (1994), pages 377-389). It was found that polymers which contain vinyl alcohol groups and acetal groups are particularly suitable.
First, acetal polymers of aliphatic aldehydes having an unsubstituted lower alkyl group and vinyl alcohol/vinyl acetate copolymers were described (U.S. Pat. No. 2,179,051, EP-A-0 216 083 and DE-A-14 47 957). However, such binders give rise to problems during the development of printing plates because the proportion of hydrophilic groups in the polymer is too low. If, however, the degree of acetalation is chosen to be low in the case of these polymers, in order to realize sufficient developability through a high proportion of vinyl alcohol, the press life is too short.
In order to achieve an improvement, partly acetalated vinyl alcohol/vinyl acetate copolymers were provided with hydrophilic or alkali-soluble groups by special reactions. In DE-A-36 44 162 and EP-A-0 274 075, aldehydes containing hydroxyl groups are used in addition to aldehydes having hydrophobic groups (e.g. alkyl or aryl groups) for the acetalation. However, this structural change does not lead to a substantial improvement in the developability or the press life.
In DE-A-20 53 363, DE-A-20 53 364 and EP-A-0 048 876, sulfonylurethane groups were introduced into the polyvinyl acetals in order to improve the developability. However, the low acidity of these groups requires developers having a high solvent content. Moreover, micro elements of printing plates produced in this manner have very little adhesion so that they are removed too rapidly by abrasion in the printing process.
DE-A-22 45 433 describes binders which can be prepared by acetalation of saponified copolymers of vinyl acetate and a monomer carrying carboxyl groups, such as crotonic acid or acrylic acid. For the acetalation, aromatic aldehydes are disclosed. However, this type of binder leads to systems having low photosensitivity, poor developability and a short press life when used for printing plates.
In EP-A-0 211 391 and EP-A-0 152 819, the carboxyl groups are introduced by reacting separately prepared acetals of aliphatic aldehydes with polyvinyl alcohol with intramolecular, cyclic anhydrides of dicarboxylic acids. However, the necessary cost of the synthesis is considerable since, in addition to the acetalation, there is the reaction with the anhydrides, which is possible only in aprotic solvents. Furthermore, the photosensitivity of the mixtures prepared from these binders is still too low.
EP-A-0 208 145 describes binders which are prepared in a three-stage synthesis starting from polyvinyl alcohol: acetalation with aliphatic aldehydes, reaction with intramolecular, cyclic anhydrides of dicarboxylic acids and partial esterification of the carboxyl groups with substituted alkyl halides. In spite of the high cost for the binder synthesis, an improvement of the photosensitivity of the layers produced therefrom is desirable.
The cost of the multistage syntheses can be avoided if, as described in WO 93/03068 and U.S. Pat. No. 5,169,897, polyvinyl alcohol is reacted with aliphatic aldehydes and aliphatic aldehydes containing carboxyl groups or aromatic aldehydes containing carboxyl groups. However, the photosensitive mixtures prepared therefrom have an unfavorable relationship with regard to developability and sensitivity. Moreover, the aldehydes containing carboxyl groups give rise to high costs.
EP-A-0 752 430 describes acetal polymers which can be prepared by reacting polyvinyl alcohol with aliphatic aldehydes and aldehydes having a xe2x80x94Xxe2x80x94N(R1)COxe2x80x94Yxe2x80x94COOH side group. The cost for the preparation of such polymers is however high since the special aldehydes are very expensive, the use of organic solvents cannot be dispensed with in the synthesis and a precipitation step using a nonsolvent is required for isolating the binders.
In spite of this intensive research in the area of photosensitive compositions for printing plates, improvements appear to be desirable in the case of all existing compositions, in particular with regard to their developability and the sensitivity to press room chemicals. Furthermore, either many of the compositions have a large number of components, some of which are expensive, or the preparation of their main components is associated with a high synthesis cost, both of which prevent commercial use.
It was therefore the object of the present invention to provide photosensitive compositions which are used in the production of printing plates, containing a binder whose preparation manages with as small a number of synthesis steps as possible compared with the binders described in the prior art, and which nevertheless has the same physical properties, or physical properties improved in individual areas, in photosensitive compositions. In particular, improved acceptance of the printing ink and/or a longer press life of the corresponding printing plates is to be achieved in comparison with the compositions described in the prior art, while retaining the good developability in developers which contain only small amounts of organic solvents, salts and wetting agents, the high photosensitivities and the good image resolutions, and the compositions should moreover be compatible with the other products used in the processing of printing plates.
This object is achieved by a photosensitive composition comprising:
(i) at least one, preferably one, diazonium polycondensate or at least one, preferably one, system capable of free radical polymerization and comprising photoinitiators and unsaturated compounds which are capable of free radical polymerization or at least one, preferably one, hybrid system comprising a diazonium polycondensate and a system capable of free radical polymerization and comprising photoinitiators and unsaturated compounds which are capable of free radical polymerization,
(ii) at least one, preferably one, binder and optionally, one or more exposure indicators, one or more dyes for increasing the image contrast and one or more acids for stabilizing the photosensitive composition, characterized in that the binder essentially, preferably exclusively, comprises units A, B, C and D, A being present in an amount of from 0.1 to 25 mol %, preferably from 1 to 18 mol %, and corresponding to the formula 
in which R1 is a hydrogen atom, an aliphatic hydrocarbon radical having 1 to 4 carbon atoms which may be unsubstituted or substituted with at least one carboxyl group, sulfonic acid group, hydroxyl group or at least one halogen atom, or an aromatic radical which may be unsubstituted or substituted with at least one alkyl group, carboxyl group, sulfonic acid group, hydroxyl group or at least one halogen atom, preferably a methyl group, B being present in an amount of from 0.1 to 60 mol %, preferably from 20 to 55 mol %, and corresponding to the formula 
C being present in an amount of from 10 to 80 mol %, preferably from 25 to 65 mol %, and corresponding to the formula 
in which R2 is a hydrogen atom, an alkyl radical having from 1 to 10 carbon atoms which may be unsubstituted or substituted with at least one carboxyl group, sulfonic acid group, hydroxyl group or at least one halogen atom, preferably a methyl, ethyl or propyl group, or an aryl group which may be unsubstituted or substituted with at least one alkyl group, carboxyl group, sulfonic acid group, hydroxyl group or at least one halogen atom, it being possible for this unit to be present several times with different radicals R2 independently of one another, and D being present in an amount of from 1 to 20 mol %, preferably from 2 to 8 mol %, and corresponding to the formula 
in which R3 is a hydrogen atom or alkyl radical having 1 to 10 carbon atoms, which may be unsubstituted or substituted by a carboxyl group, R3 preferably being a hydrogen atom, a methyl group or xe2x80x94CH2COOH group.
The advantages of the invention are to be seen in the fact that binders for photosensitive compositions can be prepared by very simple syntheses starting from economical, industrially available copolymers. The properties of the binders thus obtainable can be very exactly tailored by varying the molar mass and the degree of saponification of the starting copolymers, as well as the type of substituents R1 to R3.
It is thus possible to obtain tailor-made polymers which are suitable for use in photosensitive compositions based on different photocrosslinking mechanisms. The developability of such mixtures using conventional developers, in particular those which are used in the processing of offset printing plates, can be substantially improved without any ink acceptance problems occurring. Moreover, such compositions are very abrasion-resistant after the usual processing steps for the production of an offset printing plate, so that the printing plates can be used for print runs of more than 300,000 revolutions.
The first essential component of the photosensitive composition according to the invention is a polyvinyl alcohol which contains a carboxyl group bonded directly to the main chain and some of whose OH groups are acetalated.
The preparation of such binders is simple. Copolymers of vinyl alcohol, a vinyl ester and an olefinically unsaturated carboxylic acid serve as starting material.
Suitable vinyl ester components are those which are derived from C1-C4-carboxylic acids. Vinyl acetate is particularly preferred.
The unsaturated carboxylic acid may be unsubstituted at the a olefinic carbon atom or may carry an alkyl radical R3 having up to 10 carbon atoms, which is optionally substituted by a further carboxyl group. Straight-chain or branched alkyl radicals are suitable, methyl groups being particularly preferred. Preferred carboxylic acids are acrylic acid and methacrylic acid.
The acetal radicals R2 can be introduced into the polymer molecule starting from aldehydes R2CHO, cyclic trimers of aldehydes (e.g. paraldehyde), oligomeric aldehydes, (e.g. paraformaldehyde), or acetals of aldehydes R2CH(OR4)2, in which R4 is an alkyl radical. The radical R2 is preferably a branched or straight-chain alkyl radical having 1 to 10 carbon atoms. The synthesis starting from acetaldehyde, propionaldehyde and butyraldehyde or acetals thereof with lower alcohols R4OH is particularly preferred. The acetal radicals R2 within a binder may be different. As a result, the developability can be tailored exactly to the corresponding system. In general, the developability improves if the number of carbon atoms in R2 decreases. On the other hand, the ink acceptance is better in the case of longer-chain radicals. A preferred combination is acetaldehyde and butyraldehyde.
The preparation of the acetal polymers can be carried out by known processes in the presence of catalytic amounts of acid, as disclosed, for example, in EP-A-0 216 083 and DE-C-28 38 025. Preferred acids are hydrochloric acid, phosphoric acid, or organic sulfonic acids.
The reaction is preferably carried out at temperatures of from 0 to 90xc2x0 C. in a solvent for the starting polymer. Preferred solvents are water, tetrahydrofuran, 1,4-dioxane, 1,3-dioxolane, hexamethylphosphorotriamide, N-methylpyrrolidone, formamide, N,N-dimethylformamide, dimethyl sulfoxide, mixtures of water and isopropanol. Water is particularly preferred since in this case the reaction product is precipitated during the reaction. The end product is soluble in many organic solvents. In this case, the polymer solution is introduced into a nonsolvent for the polymer while stirring, in order to isolate the product in solid form, and the latter is filtered off. Suitable nonsolvents for the polymer are nonpolar solvents or water. Another procedure which is likewise feasible is to add the nonsolvent for the polymer to the synthesis solution while stirring. In both cases, thorough mixing is essential in order to obtain a reaction product which can be readily handled and to separate off the concomitantly formed byproducts as quantitatively as possible. The precipitated reaction product is separated off, washed with water and then dried.
The concentration of the reactants, based on the amount of solvent, is preferably from 3 to 25% by weight, particularly preferably from 7 to 16% by weight. The catalytic amount of added acid is preferably from 1 to 15% by weight, based on the product used. This amount of mineral acid can, but need not, be neutralized after the end of the reaction by adding molar amounts of an alkaline compound in order to prevent an acid-catalyzed deacetalation of the polymeric binders during storage or subsequent use. Suitable alkaline compounds are alkaline earth and alkali carbonates and hydroxides as well as ammonia. Sodium hydroxide and potassium hydroxide are preferred.
The second essential component of the photosensitive composition according to the invention is a diazonium polycondensate or a system capable of free radical polymerization and comprising photoinitiators and unsaturated compounds which are capable of free radical polymerization or a hybrid system comprising a diazonium polycondensate and a system capable of free radical polymerization and comprising photoinitiators and unsaturated compounds which are capable of free radical polymerization.
The diazonium polycondensate used in the photosensitive mixtures according to the invention may be any diazonium polycondensates known to a person skilled in the art. Such condensates can be prepared, for example, in a conventional manner by condensation of a diazo monomer described in EP-A-0 104 863 with a condensing agent, such as formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde or benzaldehyde. Furthermore, cocondensates which, in addition to the diazonium salt units, also contain other, nonphotosensitive units which are derived from condensable compounds, in particular aromatic amines, phenols, phenol ethers, aromatic thioethers, aromatic hydrocarbons, aromatic heterocycles and organic amides, are used. Particularly advantageous examples of the diazonium polycondensates are reaction products of diphenylamine-4-diazonium salts, which optionally contain a methoxy group in the phenyl radical carrying the diazo group, with formaldehyde or 4,4xe2x80x2-bismethoxymethyldiphenyl ether. Suitable anions of these diazo resins are in particular aromatic sulfonates, such as 4-tolylsulfonate or mesitylene sulfonate, tetrafluoroborate, hexafluoroantimonate, hexafluoroarsenate or hexafluorophosphate. The diazonium polycondensate is preferably present in the photosensitive mixtures in an amount of from 3 to 60% by weight.
The correct composition of the polymer according to the invention necessitates optimization experiments for the respective intended use. For example, the developability, the photosensitivity, the acceptance for inks, the stability during storage under high atmospheric humidity and temperature are determined as a function of the composition of the polymer in the manner known to a person skilled in the art.
The second essential component may also be a system capable of free radical polymerization. This is composed of photoinitiators which absorb in the range from 300 to 800 nm, preferably from 300 to 450 nm, and components capable of free radical polymerization. Suitable photoinitiators for the photosensitive compositions according to the invention are preferably the parent structures or derivatives of acetophenone, benzophenone, 2,4-(trichloromethyl)-1,3,5-triazine, benzoin, benzoin ethers, benzoin ketals, xanthone, thioxanthone, acridine or hexarylbisimidazole. That component of the mixture according to the invention which is capable of free radical polymerization is an acrylic or methacrylic acid derivative having one or more unsaturated groups, preferably esters of acrylic or methacrylic acid in the form of monomers, oligomers or prepolymers. It may be present in solid or liquid form, solid and viscous forms being preferred. The compounds which are suitable as monomers include, for example, trimethylolpropane triacrylate and methacrylate, pentaerythritol triacrylate and methacrylate, dipentaerythritol monohydroxypentaacrylate and methacrylate, dipentaerythritol hexaacrylate and methacrylate, pentaerythritol tetraacrylate and methacrylate, ditrimethylolpropane tetraacrylate and methacrylate, diethylene glycol diacrylate and methacrylate, triethylene glycol diacrylate and methacrylate or tetraethylene glycol diacrylate and methacrylate. Suitable oligomers or prepolymers are urethane acrylates and methacrylates, epoxide acrylates and methacrylates, polyester acrylates and methacrylates, polyether acrylates and methacrylates or unsaturated polyester resins. The photoinitiators and components capable of free radical polymerization should have a composition known to a person skilled in the art, combinations of different photoinitiators and different components capable of free radical polymerization also being advantageous. The amount by weight of the photoinitiators is preferably from 0.5 to 20% by weight and that of the components capable of free radical polymerization is from 5 to 80% by weight, based on the total solids content of the photosensitive compositions.
A combination of the diazonium polycondensates with a system capable of free radical polymerization and comprising photoinitiators and unsaturated compounds which are capable of free radical polymerization may have advantages for specific applications. Such hybrid systems preferably comprise from 1 to 50% by weight of diazonium polycondensate, from 0.5 to 20% by weight of photoinitiators and from 5 to 80% by weight of the components capable of free radical polymerization.
The exposure indicators which can be used in the photosensitive compositions according to the invention are known to a person skilled in the art. Exposure indicators from the series consisting of the triarylmethane dyes (such as Victoria Sky Blue BO, Victoria Blue R, Crystal Violet) or azo dyes (such as 4-phenylazo-diphenylamine, azobenzene or 4-N,N-dimethylaminoazobenzene) are preferred. The exposure indicators are present in the photosensitive composition in an amount of from 0.02 to 10% by weight, preferably from 0.5 to 6% by weight.
Suitable colorants for increasing the image contrast are those which are readily soluble in the solvent or solvent mixture used for coating or which can be introduced in disperse form as a pigment. The suitable contrast colorants include Rhodamine dyes, methyl violet, anthraquinone pigments and phthalocyanine dyes and pigments. The colorants are contained in the photosensitive composition in an amount of from 1 to 15% by weight, preferably from 2 to 7% by weight.
Furthermore, the composition according to the invention may contain stabilizer acids. Such stabilizing acids include phosphoric, citric, benzoic, m-nitrobenzoic, p-anilinoazo-benzenesulfonic, p-toluenesulfonic and/or tartaric acid. A mixture of a plurality of different acids is advantageous in some formulations. Phosphoric acid is preferably used as a stabilizing acid. The amount of the added acid is preferably from 0.2 to 3% by weight in the photosensitive composition.
The photosensitive composition according to the invention may furthermore contain a plasticizer. Preferred plasticizers include dibutyl phthalate, triaryl phosphate and dioctyl phthalate. The amounts of plasticizer used are preferably from 0.25 to 2% by weight in the photosensitive composition.
The photosensitive compositions according to the invention can preferably be used for the production of printing plates. Furthermore, however, they can also be used in recording materials for the production of images on suitable substrates and receiving sheets, for the production of reliefs which can be used as a printing plate, screens and the like, as photocurable finishes for surface protection and for the formulation of UV-curable printing inks.
For the production of lithographic printing plates, aluminum, as a substrate, is first roughened by brushing in the dry state, by brushing with suspensions of abrasives or by an electrochemical method, for example in a hydrochloric acid electrolyte. The plates which have been roughened and, optionally, anodically oxidized in sulfuric or phosphoric acid are then subjected to a hydrophilizing after treatment, preferably in aqueous solutions of polyvinylphosphonic acid, sodium silicate or phosphoric acid. The details of the above-mentioned substrate pretreatment are sufficiently well known to a person skilled in the art.
The subsequently dried substrates are coated with the photosensitive compositions from organic solvents or solvent mixtures so that dry layer weights preferably in the range from 0.5 to 4 g/m2, particularly preferably from 0.8 to 3 g/m2, are obtained. In some cases, the additional application of a top oxygen barrier layer to the photosensitive layer may be advantageous. This is particularly advantageous in the case of systems capable of free radical polymerization and in the case of the hybrid systems comprising diazonium polycondensate and systems capable of free radical polymerization. The polymers suitable for the top layer include polyvinyl alcohol, polyvinyl alcohol/polyvinyl acetate copolymers, polyvinylpyrrolidone, polyvinylpyrrolidone/polyvinyl acetate copolymers, polyvinyl methyl ether, polyacrylic acid and gelatin. The thicknesses of the top oxygen barrier layer are preferably from 0.1 to 4 g/m2 and particularly preferably from 0.3 to 2 g/m2.
The printing plates thus produced are exposed and developed in the manner known to a person skilled in the art. The developed plates are usually treated with a preservative (xe2x80x9cgummingxe2x80x9d). The preservatives comprise aqueous solutions of hydrophilic polymers, wetting agents and further additives.
It is furthermore advantageous to increase the mechanical strength of the printing layers by a heat treatment or combined use of heat and UV light for specific applications. For this purpose, before this treatment the developed plate is first treated with a solution which protects the non-image parts so that the heat treatment does not cause these parts to accept ink. A suitable solution for this purpose is described, for example, in U.S. Pat. No. 4,355,096.
The invention is explained in more detail below with reference to embodiments, but without being restricted thereby.